1. Field of the Invention
This invention relates generally to the FCC process. More specifically, this invention relates to the separation of catalyst and gaseous materials from a mixture thereof as it is discharged from the riser.
2. Description of the Prior Art
The FCC process and ballistics separation devices, or vented risers as they are more commonly referred to, are well known in the prior art. The FCC processes, as well as vented risers, are fully described in U.S. Pat. Nos. 4,701,307 and 4,792,437, the contents of which are hereby incorporated by reference. An enclosed vented riser for an FCC process discharges a mixture of catalysts and gaseous materials outwardly from the open end of a riser into a reactor vessel. In its basic form, the structure of a vented riser consists of a straight conduit section at the end of the riser and an opening that is directed upwardly into a reactor vessel with a number of cyclone inlets surrounding the outer periphery of the riser near the open end. The apparatus functions by shooting the high momentum catalyst particles past the open end of the riser. A quick separation between the catalyst and the vapors occurs due to the relatively low density of the vapors which can quickly change direction and turn to enter the inlets of the cyclones near the periphery of the riser while the heavier catalyst particles continue along a straight trajectory that is imparted by straight section of the riser conduit.
It has long been known that vented risers require a large open volume in the reactor vessel for operation. The large volume has been necessitated in part by the arrangement of the cyclone inlets relative to the reactor riser. These arrangements have included extended inlet nozzles into the cyclones as well as the use of an annular chamber defined by a cup that surrounds the riser and communicates with the cyclone inlets. Cyclones for separating particulate material from gaseous materials are well known to those skilled in the art of FCC processing. Cyclone arrangements and modifications thereto are generally disclosed in U.S. Pat. Nos. 4,670,410 and 2,535,140. The cup and the extended inlets of the cyclones feed gaseous materials containing a relatively low loading of catalyst particles tangentially into the barrel of a cyclone. Tangential entry of the gaseous materials and catalyst creates a spiral flow path that establishes a vortex configuration in the cyclone so that centripetal acceleration associated with an outer vortex causes catalyst particles to migrate towards the outside of the barrel while the gaseous materials enter an inner vortex for eventual discharge through an upper outlet. The heavier catalyst particles accumulate on the side wall of the cyclone barrel and eventually drop to the bottom of the cyclone and out via an outlet and a dip leg conduit for recycle through the FCC arrangement. The purpose of the collector cup and extended cyclone inlets is to prevent catalyst particles from short circuiting the outer vortex of the cyclone and thereby increasing the loading of catalysts particles discharged through the vapor outlets of the cyclones.
It is highly desirable to reduce the transverse cross-section required to contain the vented riser arrangement and FCC separation vessels in general. Larger vessels and the complicated inlet tube arrangements associated with previous vented riser designs add to the overall cost of the system. Moreover, the additional cross-section required for prior art vented riser designs add to the total volume in the reactor vessel which increases the residence time of hydrocarbons located therein and, generally, has adverse effects on the selectivity of the catalytic cracking and the production of desired products.